Switching devices such as driver circuits are often used in electrical circuits to drive capacitive loads. The electrical current switched at the outputs of these circuits is subject to disturbances caused by the capacitance of the driven load. Inductance also plays a role in the generation of output current disturbance. In the case of driver circuits which are incorporated into integrated circuit chips and designed to drive off-circuit capacitive loads, inductances are encountered in the transmission line connecting the load to the integrated circuit, and in the on-chip wire bonding. The disturbances in the output current caused by these inductances and the capacitance of the driven load are measured in terms of a change in output current with respect to time (di/dt).
The capacitance of an off-chip load is typically much higher than that encountered on-chip, and its value is often unknown. Thus large, low impedance drivers are required to drive such unknown, and potentially large, capacitive loads. For example, the capacitive load of an off-chip device may be only on the order of 5-10 picofarads or it can be up to around 200 picofarads. High speed devices for driving such loads are desired for obvious reasons, but faster switching speeds come at the expense of instantaneous changes in current in the output, manifested in the form of current spiking (increased di/dt). Rapid turn on and turn off of high speed, large drivers often result in unacceptably large di/dt disturbances.
Thus, it is an object of the present invention to provide a low impedance current transition rate control circuit which controls the rate at which the output current switches state, so as to prevent excessive current spiking on the output, by providing a novel slew control circuit which controls the turn on of large driving devices in CMOS off-chip driver (OCD) circuitry. It is a further object of the present invention to provide such a device in a circuit which consumes little DC power, requires little on-chip space, and is effective in regulating di/dt at its output, and which is immune to effects of random process variations on output driver noise and circuit delay.